Moving encrypted emails to folder fails

* src/folder-events.cpp: Search via searchMailsByUUID and use first
                            mail found if getMailByUUID fails

For mails that don't have a GPGOL_UID property get_unique_id returns
the Message-ID and no longer null. Calling getMailByUUID will fail. In
thats case we fallback to calling the slower searchMailsByUUID as that
will call get_unique_id for each mail, so it finds those mails via the
Message-ID and moving works again.

• GnuPG-Bug-Id: T7712